The invention relates to a vibrating table for compacting masses by vibration--in particular, concrete--whose principal plane in operation is arranged largely horizontally, and which carries a mold, containing a mass to be compacted, and on which is supported a driven (exciting) shaft, that is provided with at least one unbalanced mass for the creation of an excitation force.
Here, by vibrating table is to be understood, within the meaning of the invention under consideration, not only a table in the classical sense of the word, i.e. a device possessing a continuous table top, but also to be understood by this term are all those structural shapes using the mold containing the concrete to be compacted as a component part of the vibrating table. Thus, for example, there are vibrating tables known that possess only one frame, resting on columns, to carry the mold, or that manage completely without frames and require an appropriately rigid mold, which is supported on two stands, or such like. The concept of vibrating table, within the meaning of invention now under consideration, should, therefore, include in general all devices suitable to vibrate a mold that is placed atop them. Furthermore, the invention relates to a process for compacting concrete.
In many fields of technology it is necessary to compact masses held in containers. Here, for example, we might be dealing with bulk goods whose volume is to be reduced; in particular, however, this machine requirement is present when concrete must be poured into molds for the production of prefabricated compound units. In order to obtain high strength values, it is absolutely necessary to achieve high compaction before the concrete sets. In ready-mix cement plants various processes and devices are utilized to compact fluid concrete, and essentially they are all based on the fundamental concept of impressing the mold containing the cement to be compacted using an oscillating or rotating motion, which results in shearing strain in the cement. In this manner the cement is deaerated, and the cement correspondingly compacted.
The method of impressing the liquid cement with a motion over the mold walls is called jolting, vibrating, or shaking. The devices used to accomplish this are called, depending upon the chief frequency range and oscillation amplitude utilized, vibrating tables or vibrating girders, and, from time to time, also called jolting tables. Within the meaning of this invention, these terms shall be understood as synonyms, i.e., within the meaning of the invention the sole decisive point is that a recurrent motion runs through a container holding a mass to be compacted, in particular a mold that contains concrete.
The vibrating tables used in concrete factories operate either with so-called unbalanced exciters, in which a rotating unbalanced mass produces an excitation force, which in turn causes the table top and the mold mounted atop it to vibrate, or with linear drives, as, for example, in hydraulic cylinders, or the like, which are operated periodically with a corresponding control. Known and generally utilized unbalanced exciters consist of a shaft, onto which a weight is attached eccentrically, and which is mounted on a vibrating table. The shaft is driven either with electric, pneumatic, or hydraulic power, but can also be driven using V belts, or the like.
What has gained acceptance as state of the art in the linear drives is to compact by vibration in two directions, perpendicular to each other on the table plane, i.e., on the horizontal. For this reason two hydraulic cylinders with a controlling device, two thrust crank assemblies, or the like, are necessary. The vibrating motion is performed consecutively in time, i.e., first, for example, it is vibrated in the direction of the X-axis, and then in the direction of the Y-axis. The vibrating tables operating and/or excited in the horizontal plane are also known as horizontal vibrating tables. A paper regarding optimum vibration parameters for such vibrating tables and the obtainable compaction results can be found in Bauzeitung 1973, 10th volume, pages 510 ff.
In order to obtain the required compression in the concrete masses compacted utilizing horizontal vibrating tables, it is necessary to produce waves possessing a high density of energy in the fluid concrete mass. Here the majority of the state of the art vibrating tables are excited using high frequencies, since the density of energy is proportional to the square of frequency. The generally utilized frequency range lies from 25 to 300 Hz, i.e., within the range of audibility. Therefore, there is a loud noise produced, posing a health hazard for those working in the concrete plants. Therefore, great efforts have been undertaken to achieve the same compression effect, but using a frequency range under 15 Hz, i.e., a frequency range that can scarcely be detected by the human ear. Thus, for example, devices have been built that operate in the range from 3 to 10 Hz. However, in order to achieve the minimum acceleration required for concrete compaction, to balance the lower frequencies, higher amplitudes are required--for example, at a frequency of 5 Hz, an amplitude of 5-6 mm is required. Since the concrete prefabricated compound units, and thus the total mass of the form holding the liquid concrete, are very large, for example, in the range of 10-20 t, the creation of such parameters is difficult, due to the high forces that must be introduced.
In addition there exist vibrating tables in which the mold, that is atop the table surface and that contains the concrete to be compacted, is vibrated vertically. Here, this can lead to jarring blows when molds are used that are merely frictionally connected to the table, if, namely, the acceleration of the mold is greater than 1 g. In molds that are form-locked with the table, jarring blows can indeed be avoided. However, to obtain good compression effects, high frequencies are required, which, with the already described considerable noise nuisance and health hazards, are all part of the job for those working in the concrete plants.
Printed specification of German patent (DE-PS) 4 116 647 describes a vibrating device, which is supplied with four horizontally placed excitation shafts, each with a weight provided eccentrically, i.e., supplied with an unbalanced mass. Each of the four excitation shafts has its own drive, which is connected to a control device. One of the four drives operates as a master drive, and the phase relationship of the unbalanced masses of the three other drives is controlled in relation to the first drive, so that via the superposition of the centrifugal forces of the total four circulating unbalanced masses, a resulting excitation force vertical direction (Z-axis) can be created, whose amplitude can be affected relative to one another by regulating the phase relationship of at least two circulating unbalanced masses.
A disadvantage of this embodiment is the relatively high structural costs, which are solely utilized to create an excitation force in one direction and are consequently merely a vibration pattern, which leads to partially unsatisfactory compacting results.
While the vertically operating vibrating tables are inferior to the horizontally operating vibrating tables as regards noise development, the hitherto known horizontal vibrating process is encumbered by the disadvantage that compacting time is greater than in the vertical process and/or vibrating tables.
Therefore, the invention is based upon the object of designing a horizontal vibrating table that will allow a high compression effect within a short period of time, and that has a low noise nuisance, and which is characterized by a simple structural design. In addition, the invention is based upon the object of further developing a horizontal vibrating table whereby, depending upon the specifications, a plurality of vibration modes are possible, in order to favor a good compression of concrete and/or other masses to be compacted. In this connection, the basic idea, as foreseen in the invention, sees as a solution in a horizontal vibrating table, in accordance with its class, that at least one excitation shaft shall be provided vertically.
Also, in a preferred embodiment of the invention several, for example, two or four, excitation shafts could be provided on the vibrating table, symmetrical to each other with respect to rotation.
In general, with this basic idea, in accordance with the invention, i.e., to provide vertically placed excitation shafts with unbalanced masses, we are confronted with the problem that, due to the rotating masses, a shifting of the center of gravity of the total system occurs, which produces moments around the Y-axis or the X-axis, which in turn causes the appearance of vertical amplitudes. A disadvantage of the vertical vibration created by said moments is, in particular, that here we are dealing with torsional vibration around the X-axis or Y-axis, whereby in the nature of things the vertical amplitudes in the outer zone of the mold are greater than in the center of gravity -and/or in axial proximity, which leads to the fact that the compression effect in the outer zone of the mold is relatively good, but insufficient in the proximity of the center of gravity or the axes.
With the appearance of vertical vibration components, in addition to the non-uniform compression effect, there is to a large extent the additional serious problem that the natural frequency of the total system in the vertical direction can lie so close to the excitation frequencies that the total system, consisting of vibrating table, mounted mold with concrete for compacting, and the total weight of the excitation shafts, can be caused to be in resonance, whereby the resulting large amplitudes can then slosh the concrete out of the mold, and cause damage to the vibrating table.
Therefore, as regards its part as a device, as part of the object, to a large extent, the invention is based upon the object of designing a horizontal vibrating table with vertically placed excitation shafts, in accordance with the invention, so that the said vibrations will be suppressed by the vertical vibration components.
Viewed in terms of vibration technology, this part of the object is comparable to the task of designing the total system, consisting of horizontal vibrating table, mounted mold that contains concrete to be compacted, and vertically placed shafts on the vibrating table, in such a way that, in the vertical direction, no resonances occur with the frequencies under consideration, i.e., in the frequency range below the threshold of audibility.
The solution to the second part of the object with the vibrating table, in accordance with its class, fitted with vertically situated excitation shaft(s), in accordance with the invention, is characterized in that the table is supported in such a fashion that it possesses in the principal plane two degrees of freedom (X-, Y-direction). However, in the direction of the gravitational force field, it is essentially rigidly supported.
In regard to the part in accordance with the device, the invention is based upon the object of further developing a process to vibrate (shake, vibratory compaction) of concrete in a mold to be compacted, so that the compression effect in comparison to the state of the art is improved, or at least preserved, whereby, however, noise production is diminished, and the process can be performed on a vibrating table that is structurally comparatively simple.
In regard to the procedural part, the invention is characterized in that the mold, which contains the concrete to be compacted, is excited on a vibrating table with circular shaking motions in the horizontal principal plane of the table.
Surprisingly it has been proven that in no way is it necessary to vibrate in directions perpendicular to one another in order to obtain sufficient shearing forces in concrete for a sufficient compression effect. A uniform and circular motion to create shearing strain in the concrete to be compacted is sufficient to obtain the desired sufficient compression of the concrete.